英语时差：光速与星际高速路（上）

If you’re driving on an interstate highway, the only thing keeping you from going faster than the speed limit is your sense of civic duty, or perhaps that police car up ahead. There’s one speed limit, however, that most scientists agree nothing will ever break. That’s the speed of light: 186,280 miles per second. As it travels through deep space, light moves at exactly this speed, no more and no less. The speed of light is so reliable, we even use it to measure distances between the stars. If it takes starlight fifty years to reach us, we say that this star is fifty light-years away. You might be surprised to learn, however, that light itself does not always travel at the speed of light. Our interstate highways generally have a higher speed limit, but once you enter a city or residential street, the speed limit lowers.